Microwave components are assembled into microwave circuit packages that provide convenient structures for handling, installing, and connecting the microwave components to external circuits on a printed circuit board. A microwave circuit package typically includes multiple layers of dielectric material (e.g., plastic or ceramic material) that support respective sets of high-frequency signal traces, direct-current (DC) signal traces, and ground traces. Traces on different dielectric material layers typically are interconnected by electrically conducting vias that extend through the dielectric material layers. Although other connection methods are possible, cost and compatibility considerations often dictate that the traces of a microwave circuit package be connected to the microwave components that are mounted in the microwave circuit package by bond wires.
In microwave circuits, a source is interconnected to a load (e.g., an integrated circuit chip) by a signal path that is modeled as a transmission line. In general, the source and the load are impedance-matched to the nominal impedance of the transmission line in order to minimize losses and reflections and achieve maximal power transfer from the source to the load. Any transition in the signal path (e.g., any change in the electrical or physical characteristics of the signal path) introduces discontinuities in the impedance of the signal path, causing signal reflections that degrade the integrity of the transmitted signal and that reduce the power transferred to the load. Signal vias and bond wires are transitions that behave as parasitic inductances that cause significant reflections and significant degradation in the transmitted signal integrity, especially at frequencies in the GHz range and higher.
Different methods have been proposed for compensating the parasitic inductances of signal vias and bond wires. In one approach, the series inductance of ball-grid-array (BGA) transitions is compensated by placing ground vias around the signal via to form a quasi-coaxial structure along the BGA transition that increases the shunt capacitance to ground. The diameters and the spacings between the signal and ground vias are adjusted to change the shunt capacitance to a level that approximately matches the impedance of the transition to the impedances of the other sections of the signal path. In another approach, the series inductance of a bond wire is compensated by adding excess capacitance to the end of the signal trace on the microwave circuit package that is connected to one end of the bond wire.
Microwave circuit interconnections that incorporate such approaches to compensation of inductive parasitics are capable of impedance-matching via and bond wire transitions to the nominal impedance of the signal path sufficiently to achieve reasonable performance (e.g., −15 dB return loss) over frequencies ranging from DC up to about 50 GHz. These approaches to the compensation of inductive transitions, however, cannot match the impedance of these transitions to the nominal impedance of the signal path with sufficient accuracy to achieve high performance signal transmission (e.g., −20 dB return loss or better) at frequencies at or above 20 GHz, which is required for instrumentation and high quality commercial applications.